This invention relates to a relief valve for a tilt device and more particularly to an improved relief valve for a hydraulic circuit.
Relief valves are utilized for a wide variety of purposes and in a wide variety of applications. For example, a relief valve or a series of relief valves are employed in the hydraulic tilt and trim units for marine propulsion devices so as to protect against excess pressure in the systems. Although such devices have a wide variety of application, the conventional prior art type of relief valve has certain disadvantages which will be discussed.
In a conventional hydraulic relief valve, a movable valve member is biased into engagement with a valve seat and holds a communication passageway closed between the pressure side of the hydraulic circuit and a relief side. When the pressure on the pressure side exceeds the spring pressure or biasing pressure on the valve member, the valve member will open and relieve the pressure until a normal pressure is returned at which time the valve will again seat. Although in theory this is a very practical type of device, the fact that the conventional relief valve must be held against fluid pressure in its closed position, means that this type of valve has its seating pressure inversely related to the pressure in the hydraulic circuit. As a result, some leakage tends to occur with these valves. In addition, the conventional type of relief valve is prone to having foreign matter be trapped between the valve member and its seat that can give rise to additional leakage.
These problems may be best understood by reference to FIGS. 1, 2 and 3 wherein a conventional type of relief valve is shown in FIG. 1. FIG. 2 shows the leakage rate of this type of valve in relation to pressure by the curve A while FIG. 3 shows the sealing force S in relation to pressure, against by the curve A.
Referring specifically to FIG. 1, a conventional prior art type of relief valve is indicated by the reference numeral 11 and includes a valve housing 12 in which a high pressure passageway 13 is provided that communicates with the appropriate pressure conduit in which pressure is to be controlled. A valve seat 14 is formed at the end of the passageway 13 and a ball type check valve 15 is normally urged into engagement with the valve seat 14 by a coil compression spring 16 and spring retainer 17. When the pressure in the passageway 13 exceeds the seating pressure applied on the valve 15 by the spring 16, the valve 15 will open to permit flow into a chamber 18 and be discharged through a return passgeway 19 to the reservoir or a lower pressure area. Once the pressure has been relieved, the valve element 15 will again seat.
As may be clearly seen in FIG. 3, the seating force S is inversely related to the pressure P in the conduit 13. This is because the spring 16 must act against the pressure to hold the valve in the closed position. As a result of this, at a predetermined pressure less than the pressure P.sub.o, certain leakage will occur which leakage increases as the pressure increases until the point P.sub.o at which time the valve will fully open. As a result, there is the likelihood of leakage and pressure loss before the desired pressure to be relieved exists.
In addition to these difficulties, foreign material may become lodged in the area of the valve seat 14 and prevent full seating which can give rise to greater leakage problems.
It is, therefore, a principal object of this invention to provide an improved relief valve.
It is a further object of this invention to provide a relief valve wherein the seating pressure is directly related to the pressure in the conduit where the pressure is to be relieved until the relief pressure is reached.
It is a further object of the invention to provide an improved relief valve of the type wherein leakage of the valve due to foreign materials can be substantially reduced.